Coated optical fiber and optical fiber coating system including a fast-gelling primary coating

ABSTRACT

The present invention provides optical fiber coating systems and coated optical fibers. According to one embodiment of the invention, a coated optical fiber includes an optical fiber having a core and a cladding; and a primary coating encapsulating the optical fiber, the primary coating having a Young&#39;s modulus of about 5 MPa or less, the primary coating being the cured reaction product of a primary curable composition having a gel time less than about 1.4 seconds at a UV intensity of 3.4 mW/cm 2 .

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to optical fiber, and moreparticularly to coating systems for optical fiber and coated opticalfibers.

2. Technical Background

Optical fiber has acquired an increasingly important role in the fieldof telecommunications, frequently replacing existing copper wires. Thistrend has had a significant impact in all areas of telecommunications,greatly increasing the amount of data that is transmitted. Furtherincrease in the use of optical fiber is foreseen, especially in metroand fiber-to-the-home applications, as local fiber networks are pushedto deliver an ever-increasing volume of audio, video, and data signalsto residential and commercial customers. In addition, use of fiber inhome and commercial premise networks for internal data, audio, and videocommunications has begun, and is expected to increase.

Optical fiber is typically made of glass, and usually has a polymericprimary coating and a polymeric secondary coating. The primary coating(also known as an inner primary coating), is typically applied directlyto the glass fiber, and when cured forms a soft, elastic, compliantmaterial encapsulating the glass fiber. The primary coating has a lowYoung's modulus, and serves as a buffer to cushion and protect the glassfiber during bending, cabling or spooling. The secondary coating (alsoknown as an outer primary coating) is applied over the primary coating,and acts as a tough, protective outer layer that prevents damage to theglass fiber during processing, handling and use.

As the demand for optical fibers has increased, so has the desire toimprove the processes used to make them. One common trend in the opticalfiber industry has been the desire to draw optical fibers at increasedspeeds, thereby increasing the throughput of optical fiber manufacturingplants. However, the draw speed can be rate-limited by the step ofcuring the conventional polymeric coatings used to protect the fiber.One sign that a coating is applied to an optical fiber at a rateexceeding the coating's maximum draw speed is the presence of defects inthe cured coating. There remains a need for optical fiber coatings thatcan be fully cured at higher draw speeds using standard optical fibercoating curing processes.

SUMMARY OF THE INVENTION

One embodiment of the present invention relates to a coated opticalfiber including an optical fiber having a core and a cladding; and aprimary coating encapsulating the optical fiber, the primary coatinghaving a Young's modulus of about 5 MPa or less, the primary coatingbeing the cured reaction product of a primary curable composition havinga gel time less than about 1.4 seconds at a UV intensity of 3.4 mW/cm².

Another embodiment of the present invention relates to a method forcoating an optical fiber including the steps of providing a bare opticalfiber; coating the optical fiber with a primary curable compositionhaving a gel time less than about 1.4 seconds at a UV intensity of 3.4mW/cm²; and curing the primary curable composition to form a primarycoating encapsulating the optical fiber, the primary coating having aYoung's modulus less than about 5 MPa.

Another embodiment of the present invention relates to a curablecomposition having a gel time less than about 1.4 seconds at a UVintensity of 3.4 mW/cm², wherein a substantially cured reaction productof the curable composition has a Young's modulus less than about 5 MPa.

The coated optical fibers, methods, and curable compositions of thepresent invention result in a number of advantages over prior art coatedoptical fibers, methods, and curable compositions. For example, thecurable compositions of the present invention can be cured at high ratesof speed to provide defect-free coated optical fibers, enabling theskilled artisan to increase the throughput of optical fibermanufacturing processes.

Additional features and advantages of the invention will be set forth inthe detailed description which follows, and in part will be readilyapparent to those skilled in the art from the description or recognizedby practicing the invention as described in the written description andclaims hereof, as well as in the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are merely exemplary of theinvention, and are intended to provide an overview or framework forunderstanding the nature and character of the invention as it isclaimed.

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings are not necessarily to scale,and sizes of various elements may be distorted for clarity. The drawingsillustrate one or more embodiment(s) of the invention, and together withthe description serve to explain the principles and operation of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a coated optical fiber according oneembodiment of the present invention; and

FIG. 2 is a plot showing the spectral output of the UV source used ingel time determinations.

FIG. 3 is a typical plot of G′ and G″ vs. time used in determining thegel time of a curable composition;

FIG. 4 is a schematic cross-sectional view of a dynamic photo-rheometryapparatus; and

FIG. 5 is a plot showing the spectral output of the UV source used inspectroscopic cure speed determinations.

FIG. 6 is a typical plot of % conversion vs. time used in determiningthe spectroscopic cure speed of a curable composition.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

One embodiment of the present invention relates to a coated opticalfiber. An example of a coated optical fiber is shown in schematiccross-sectional view in FIG. 1. Coated optical fiber 20 includes anoptical fiber 22 having a core 24 and a cladding 26; and a primarycoating 30 encapsulating the optical fiber. Coated optical fiber 20 alsoincludes a secondary coating 32 encapsulating the primary coating 30. Inthe coated optical fiber of FIG. 1, the primary coating 30 is applieddirectly to the surface of the optical fiber, and the secondary coating32 is applied directly to the surface of the primary coating. As theskilled artisan will appreciate, in alternative embodiments of theinvention, a thin (e.g., less than 10 μm in thickness) layer of anothercoating may be formed between the optical fiber and the primary coating,and/or between the primary coating and the secondary coating.

The optical fiber 22 is an uncoated optical fiber including a core and acladding, as is familiar to the skilled artisan. The uncoated opticalfiber may be a single mode fiber or a multimode fiber. The optical fibermay be adapted for use as a data transmission fiber (e.g., SMF-28®,LEAF®, and METROCOR®, each of which is available from ComingIncorporated of Coming, N.Y.). Alternatively, the optical fiber mayperform an amplification, dispersion compensation, or polarizationmaintenance function, or may be used in short lengths in couplingoptical devices. The skilled artisan will appreciate that the coatingsdescribed herein are suitable for use with virtually any optical fiberfor which protection from the environment is desired.

In coated optical fiber 20, optical fiber 22 is surrounded by a primarycoating 24. In order to provide adequate cushioning and bend protectionfor the optical fiber, primary coating 24 has a Young's modulus about 5MPa or less. Desirably, the primary coating has a Young's modulus ofabout 2 MPa or less. In certain especially desirable embodiments of thepresent invention, the primary coating has a Young's modulus of about1.5 MPa or less, about 1 MPa or less, or even about 0.8 MPa. As usedherein, the Young's modulus of a primary coating is measured using atensile testing instrument (e.g., a Sintech MTS Tensile Tester, or anInstron Universal Material Test System) on a sample of material shapedas a film between about 0.003″ (76 μm) and 0.004″ (102 μm) in thicknessand about 1.3 cm in width, with a gauge length of 5.1 cm, and a testspeed of 2.5 cm/min. Desirably, the primary coating has a glasstransition temperature more negative than about −10° C.

Primary coating 24 is the cured reaction product of a primary curablecomposition having a short gel time. For example, in one embodiment ofthe present invention, the primary coating is the cured reaction productof a primary curable composition having a gel time less than about 1.4seconds at a UV intensity of 3.4 mW/cm² between 325 nm and 425 nm (usinga mercury vapor lamp operating near 365 nm, having the spectral outputshown in FIG. 2). Desirably, the primary curable composition has a geltime less than about 1.2 seconds at a UV intensity of 3.4 mW/cm² between325 nm and 425 nm. In another embodiment of the present invention, theprimary coating is the cured reaction product of a primary curablecomposition having a gel time less than about 0.6 seconds at a UVintensity of 8.5 mW/cm² between 325 nm and 425 nm. Desirably, theprimary curable composition has a gel time less than about 0.45 secondsat a UV intensity of 8.5 mW/cm² between 325 nm and 425 nm.

As used herein, the gel time of a curable composition is defined as thetime, under an exposure to a UV source of a given intensity, for asample of the curable composition formed as a layer 25 μm in thicknessto achieve a viscous modulus equal to its elastic modulus. This is showngraphically in FIG. 3. As time increases, the curable composition iscured by the exposure to UV, the viscous modulus G′ increases, and theelastic modulus G″ increases. At the gel time (t_(gel) in FIG. 3), G′and G″ have become equal. The gel time is measure of how quickly thecurable composition forms a physically stable polymeric network, and sois highly related to the processability of the curable composition.

The gel time of a curable composition is determined using dynamicphoto-rheometry, as described in Lee et al., “A Theologicalcharacterization technique for fast UV-curable systems,” Progress inOrganic Coatings 38, 193-97 (2000), which is hereby incorporated hereinby reference in its entirety. A schematic view of a suitablephoto-rheometery apparatus 40 is given in FIG. 4. Rheometer body 41includes an opening 44 and a passage 46 through which a UV light source56 is coupled (through liquid light guide 57) to a mirror 48, which isangled to couple light from UV light source 56 down the shaft 50 ofrheometer body 41. A quartz cylinder 52 is attached to the proximal end58 of rheometer body 41, and the distal end 60 of rheometer body 40 iscoupled to a transducer 62. A suitable rheometer body 41 is theRheometric RDA-II, available from Rheometric of Piscataway, N.J. The end64 of the quartz cylinder 52 is mounted above an actuator 66, with a gap68 formed therebetween. A sample 70 of the curable composition is filledinto gap 68. While the sample 70 is cured using UV light from UV lightsource 56, the actuator 66 rotationally oscillates and dynamicallyshears sample 70 between the end 64 of quartz cylinder 52 and the plate74 of actuator 66. The transducer 62 measures as a function of timeduring cure the torque exhibited by the sample in response to theoscillation of actuator 64. The transducer 62 is operatively coupled todata acquisition system 72 capable of acquiring torque and angularposition at a rate of at least about 10 Hz and providing dynamicmodulus, elastic modulus, and viscous modulus measurements continuouslyas the sample 70 cures.

In the gel time determination of the present invention, the rheometer isoperated at room temperature at a frequency of 10 Hz and an oscillatoryshear strain of about 30%. The UV light source is a GREEN SPOT UV spotcuring source, available from UV Source Inc., of Torrance Calif.,coupled to a liquid light guide configured to deliver the UV light tothe sample. The gel time determinations used to test the curablecompositions of the present invention were performed at UV intensitiesof 3.4 mW/cm² and 8.5 mW/cm ². The intensity is determined by firstmeasuring the UV intensity at the sample location using a SOLASCOPE 2000(from 4D Controls Ltd., Redruth, Cornwall, UK). The measurement was madethrough a 0.3 neutral density filter, with the SOLASCOPE reading 17mW/cm², meaning the actual unfiltered intensity at the sample locationwould be 34 mW/cm². Gel time determinations were performed with a 1.0neutral density filter in place, giving an intensity of 3.4 mW/cm², orwith a 0.6 neutral density filter in place, giving an intensity of 8.5mW/cm². The thickness of the gap 68, and therefore of sample 70 wasabout 25 μm.

Primary coating 24 is desirably the cured reaction product of a primarycurable composition having a spectroscopic cure speed of at least about150%/second as determined by FTIR. More desirably, the primary curablecomposition has a spectroscopic cure speed of at least about 170%/secondas determined by FTIR.

The spectroscopic cure speed of a curable composition is determined bymonitoring the acrylate bond conversion as a function of time using realtime FTIR. The acrylate bond conversion is measured by monitoring thedisappearance of an acrylate band at 1410 cm⁻¹, as integrated andratioed to a theoretically unchanging reference band. In the experimentsherein, the band at 1380 cm⁻¹ corresponding to a methyl group of theoligomeric backbone was used as the reference band; the skilled artisanwill select an appropriate reference band when measuring othercompositions. Films 25 μm in thickness were drawn directly on a 3-bouncediamond-coated ZnSe crystal in an ASI DURASAMPLIR accessory, and purgedwith nitrogen for 1 min. Mid-infrared spectra from 4000 cm⁻¹ to 650 cm⁻¹were collected at 6 ms intervals using a Bruker IFS 66S spectrometer for0.9 sec prior to UV exposure. UV radiation from a Lesco Mark II spotcure unit (Lightwave Energy Systems, Torrance, Calif.) was conductedthrough a liquid light guide to the sample. The UV intensity wasmeasured to be 20 mW/cm² between 250 nm and 425 nm using a SOLASCOPE2000. The spectral output of the Lesco Mark II spot cure unit is shownin FIG. 5. A shutter is used to control the dose at the sample. In thespectroscopic cure speed determinations described herein, the shutterwas closed for the first 0.9 seconds in order to provide data for thedetermination of the band ratio before exposure. The shutter was openedand the sample irradiated for 1 second, then the shutter was closed for7 seconds. Finally, the shutter was open and the sample irradiated for10 seconds. FIG. 6 is a plot of % conversion vs. time for a typicalspectroscopic cure speed determination. The % conversion is calculatedas:% conversion=band ratio at time t−band ratio before exposure/fully curedband ratio−band ratio before exposure×100%.The fully cured band ratio is the band ratio after the final 10 secondexposure. The cure speed is calculated as the rate of % conversion inthe linear region from 10% to 40% conversion.

The skilled artisan will appreciate that between different families ofcurable compositions, the relationship between gel time andspectroscopic cure speed can vary widely. For example, curablecompositions that have different chemical makeup can have very differentgel times, even if their spectroscopic cure speeds are similar. Further,as described in Gasper, S. M. et al., “Integrated Approach to Studyingthe Development and Final Network Properties of Urethane AcrylateCoatings,” Polymer Preprints 44(1), 27 (2003), which is incorporatedherein by reference in its entirety, the gel times of a series ofrelated coating is not strictly related to their cure speeds. Forexample, a series of analogous coatings having increasing cure speedscan have decreasing gel times, or even increasing gel times.

Primary coating 24 desirably has a glass transition temperature lowerthan the lowest projected use temperature of the coated optical fiber.For example, the primary coating desirably has a glass transitiontemperature less than about −10° C. In especially desirable embodimentsof the invention, the primary coating has a glass transition temperatureof about −20° C. or less. Primary coating 24 desirably has a higherrefractive index than the cladding of the optical fiber in order toallow it to strip errant optical signals away from the core of opticalfiber 22. In a typical optical fiber used for long-distance transmissionof optical signals, the refractive index values at a wavelength of 1550nm for the core and cladding are 1.447 and 1.436, respectively; as such,for typical silica based optical fibers, it is desirable that therefractive index of the primary coating of be greater than 1.44 at 1550nm. The primary coating should maintain adequate adhesion to the glassfiber during thermal and hydrolytic aging, yet be strippable therefromfor splicing purposes. The primary coating typically has a thickness inthe range of 25-50 μm (e.g., about 32.5 μm). Primary coatings aretypically applied to the optical fiber as a liquid and cured, as will bedescribed in more detail hereinbelow.

The polymeric material used as the primary coating in the presentinvention may be the cured product of a primary curable compositionincluding an oligomer and at least one monomer. As is conventional, theprimary curable composition used in forming the primary coating may alsoinclude photoinitiators, antioxidants, and other additives familiar tothe skilled artisan. In desirable embodiments of the invention, theoligomer and monomer(s) of the primary curable composition areethylenically unsaturated. In especially desirable embodiments of theinvention, the oligomer and monomer(s) of the primary curablecomposition are (meth)acrylate-based. The oligomer may be, for example,a urethane (meth)acrylate oligomer. However, as the skilled artisan willrecognize, oligomers and monomers adapted for other curing chemistries,such as epoxy, vinyl ether, and thiol-ene, may be used in accordancewith the present invention.

The skilled artisan will select monomers and oligomers that providedecreased gel times and low Young's moduli. For example, one especiallydesirable type of oligomer for use in providing curable compositionshaving decreased gel times and low moduli is a polyether urethaneacrylate oligomer having a molecular weight between 3000 and 15000Daltons. One especially desirable type of monomer for use in providingcurable compositions having decreased gel times and low moduli is amonofunctional aliphatic epoxy acrylate monomer, such aslauryloxyglycidyl acrylate (e.g., CN130 available from Sartomer Company)and phenoxyglycidyl acrylate (e.g., CN131 available from SartomerCompany). It may be desirable to use the monofunctional aliphatic epoxyacrylate monomer at a concentration of 5-40 wt %. In certain especiallydesirable embodiments of the invention, the monofinctional aliphaticepoxy acrylate monomer is present in the primary curable composition ina concentration from 10-30 wt %. Another especially desirable type ofmonomer for use in providing curable compositions having decreased geltimes and low moduli is a multifunctional (meth)acrylate. As usedherein, multifunctional (meth)acrylates have two or more polymerizable(meth)acrylate moieties per molecule. In certain desirable embodimentsof the invention the multifunctional (meth)acrylate has three or morepolymerizable (meth)acrylate moieties per molecule. Examples ofmultifunctional (meth)acrylates include dipentaerythritol monohydroxypentaacrylate (e.g., PHOTOMER 4399 available from Cognis);methylolpropane polyacrylates with and without alkoxylation such astrimethylolpropane triacrylate, ditrimethylolpropane tetraacrylate(e.g., PHOTOMER 4355, Cognis Corp.); alkoxylated glyceryl triacrylatessuch as propoxylated glyceryl triacrylate with propoxylation being 3 orgreater (e.g., PHOTOMER 4096, Cognis Corp.); and erythritolpolyacrylates with and without alkoxylation, such as pentaerythritoltetraacrylate (e.g., SR295, available from Sartomer Company, Inc.(Westchester, Pa.)), ethoxylated pentaerythritol tetraacrylate (e.g.,SR494, Sartomer Company, Inc.), and dipentaerythritol pentaacrylate(e.g., PHOTOMER 4399, Cognis Corp., and SR399, Sartomer Company, Inc.).The multifunctional acrylate is desirably present in the primary curablecomposition at a concentration of 0.05-15 wt %. In certain especiallydesirable embodiments of the invention, the multifunctional(meth)acrylate monomer is present in the primary curable composition ina concentration from 0.1-10 wt %. Another desirable type of monomer foruse in providing curable compositions having decreased gel times and lowmoduli is an N-vinyl amide, such as a N-vinyl lactam. Examples ofN-vinyl amides include N-vinyl pyrrolidinone and N-vinyl caprolactam. Itmay be desirable to use the N-vinyl amide monomer at a concentration of2-40 wt %. In certain especially desirable embodiments of the invention,the N-vinyl amide monomer is present in the primary curable compositionin a concentration from 4-25 wt %.

Desirable acrylate-terminated oligomers for use in the primary curablecompositions include BR3731, BR3741, BR582 and KWS4131, from BomarSpecialty Co.; polyether urethane acrylate oligomers (e.g., CN986,available from Sartomer Company); polyester urethane acrylate oligomers(e.g., CN966 and CN973, available from Sartomer Company; and BR7432,available from Bomar Specialty Co.); polyether acrylate oligomers (e.g.,GENOMER 3456, available from Rahn AG); polyester acrylate oligomers(e.g., EBECRYL 80, 584 and 657, available from UCB Radcure); and epoxyacrylate oligomers (e.g., CN120, available from Sartomer Company, andEBECRYL 3201 and 3604, available from UCB Radcure). Other oligomers aredescribed in U.S. Pat. Nos. 4,609,718; 4,629,287; and 4,798,852, each ofwhich is incorporated herein by reference. The above described oligomersmay be used singly, or in combination, as the skilled artisan wouldreadily appreciate. The oligomer of the primary curable composition isdesirably selected to provide the primary coating with the desired glasstransition temperature and tensile properties. One type of desirableoligomer for use in the primary curable composition is an oligomerhaving a soft block having Mn of about 4000 Daltons or greater. Examplesof such oligomers are described in U.S. patent application Ser. No.09/916,536, which is incorporated herein by reference in its entirety.Oligomers that are especially desirable for use in the primary coatingcompositions of the present invention have flexible backbones, lowpolydispersities, and low crosslink densities.

The total oligomer content of the primary curable composition may bebetween about 5 wt % and about 95 wt %. Desirably, the total oligomercontent of the primary curable composition is between about 25 wt % andabout 75 wt %. In certain embodiments of the invention, the oligomercontent of the primary curable composition is between about 40 wt % andabout 60 wt %.

The monomer component of the primary curable composition is generallyselected to be compatible with the oligomer, to provide a low viscosityformulation, and to increase the refractive index of the primarycoating. One group of suitable monomers for use in the monomer componentincludes ethoxylated acrylates, ethoxylated alkylphenol monoacrylates,propylene oxide acrylates, n-propylene oxide acrylates, iso-propyleneoxide acrylates, monofunctional acrylates, multifunctional acrylates,and combinations thereof. Especially preferred monomers includeR₂—R₁—O—(CH₂CH₃CH—O)_(n)—COCH═CH₂, where R₁ and R₂ are aliphatic,aromatic, or a mixture of both, and n=1 to 10, andR₁—O—(CH₂CH₃CH—O)_(n)—COCH═CH₂, where R₁ is aliphatic or aromatic, andn=1 to 10. Specific examples include ethylenically unsaturated monomersincluding lauryl acrylate (e.g., SR335 available from Sartomer Company,Inc., AGEFLEX FA12 available from CPS Chemical Co. (Old Bridge, N.J.),and PHOTOMER 4812 available from Cognis (Ambler, Pa.)), ethoxylatednonylphenol acrylate (e.g., SR504 available from Sartomer Company, Inc.and PHOTOMER 4003 available from Cognis), caprolactone acrylate (e.g.,SR495 available from Sartomer Company, Inc., and TONE M-100 availablefrom Dow Chemical), phenoxyethyl acrylate (e.g., SR339 available fromSartomer Company, Inc., AGEFLEX PEA available from CPS Chemical Co., andPHOTOMER 4035 available from Cognis), isooctyl acrylate (e.g., SR440available from Sartomer Company, Inc. and AGEFLEX FA8 available from CPSChemical Co.), tridecyl acrylate (e.g., SR489 available from SartomerCompany, Inc.), isobornyl acrylate (e.g., SR506 available from SartomerCompany, Inc. and AGEFLEX IBOA available from CPS Chemical Co.),tetrahydrofurfuryl acrylate (e.g., SR285 available from SartomerCompany, Inc.), stearyl acrylate (e.g., SR257 available from SartomerCompany, Inc.), isodecyl acrylate (e.g., SR395 available from SartomerCompany, Inc. and Ageflex FA10 available from CPS Chemical Co.),2-(2-ethoxyethoxy)ethyl acrylate (e.g., SR256 available from SartomerCompany, Inc.), and combinations thereof.

In certain embodiments of the invention, it may be desirable to use ahydroxyfunctional monomer in the primary curable composition. Ahydroxyfunctional monomer is a monomer that has a pendant hydroxy moietyin addition to an oligomer-reactive functionality (e.g., acrylate).Examples of hydroxyfunctional monomers including pendant hydroxyl groupsinclude caprolactone acrylate (available from Dow Chemical as TONEM-100); poly(alkylene glycol) mono(meth)acrylates, such as poly(ethyleneglycol)monoacrylate, poly(propylene glycol)monoacrylate, andpoly(tetramethylene glycol) monoacrylate (each available from Monomer,Polymer & Dajac Labs); 2-hydroxyethyl (meth)acrylate, 3-hydroxypropyl(meth)acrylate, and 4-hydroxybutyl (meth)acrylate (each available fromAldrich). The hydroxyfunctional monomer is desirably present in anamount sufficient to improve adhesion of the primary coating to theoptical fiber. For example, the hydroxyfunctional monomer may be presentin an amount between about 0.1 wt % and about 25 wt % of the primarycurable composition. Desirably, the hydroxyfunctional monomer is presentin an amount between about 0.5 wt % and about 8 wt % of the primarycurable composition. The use of the hydroxyfunctional monomer maydecrease the amount of adhesion promoter necessary for adequate adhesionof the primary coating to the optical fiber. The use of thehydroxyfunctional monomer may also tend to increase the hydrophilicityof the primary coating. Hydroxyfunctional monomers are described in moredetail in U.S. patent application Ser. No. 09/712,565, which isincorporated herein by reference.

The total monomer content of the primary curable composition may bebetween about 5 wt % and about 95 wt %. Desirably, the total monomercontent of the primary curable composition is between about 25 wt % andabout 65 wt %. In certain embodiments of the invention, the monomercontent of the primary curable composition is between about 35 wt % andabout 55 wt %.

Through variation of the oligomers, and the polyols from which they arebased, coatings having the desired properties (e.g., T_(g), modulus,elongation) can be prepared in accordance with the present disclosure.The mechanical properties of these coatings can be adjusted by thechoice of the oligomer and the monomer component. In order to providecurable compositions with a viscosity that is in a range suitable forprocessing, the viscous oligomers may be diluted with low viscosity,radiation curable monomers with which the oligomers are compatible. Incertain embodiments of the invention, it maybe desirable for theoligomers and monomers to be chosen to provide a hydrophilic primarycoating, as suggested in U.S. patent application Ser. No. 10/675,720,entitled “COATED OPTICAL FIBER AND OPTICAL FIBER COATING SYSTEMINCLUDING A HYDROPHILIC PRIMARY COATING,” which is hereby incorporatedby reference in its entirety.

In addition, according to the Fox equation, the ultimate glasstransition temperature of a cured coating will be a function of theglass transition temperatures of the components of the coatingformulation from which it is made. Thus, a desirable monomer in anoptical fiber coating would be a low viscosity material with a lowhomopolymer glass transition temperature, which can readily dissolve theoligomer and which does not negatively impact the mechanical propertiesof the cured coating. In addition to low T_(g) and suitable viscosity,the selection of the oligomer and monomer combinations may be influencedby other desirably properties for optical fibers. These additionalproperties include suitably high refractive index, good optical clarity,low oil sensitivity, high thermal and light resistance, low extractablecontent, and fast cure.

The primary curable composition may also contain a polymerizationinitiator which is suitable to cause polymerization (i.e., curing) ofthe composition after its application to an optical fiber.Polymerization initiators suitable for use in the primary curablecompositions of the present invention include thermal initiators,chemical initiators, electron beam initiators, and photoinitiators.Particularly preferred are the photoinitiators. For most acrylate-basedcoating formulations, conventional photoinitiators, such as ketonicphotoinitiating and/or phosphine oxide additives, are preferred. Whenused in the compositions of the present invention, the photoinitiator ispresent in an amount sufficient to provide rapid ultraviolet curing.

Suitable photoinitiators include 1-hydroxycyclohexylphenyl ketone (e.g.,IRGACURE 184 available from Ciba Specialty Chemical (Hawthorne, N.Y.);bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide (e.g.,commercial blends IRGACURE 1800, 1850, and 1700 available from CibaSpecialty Chemical); 2,2-dimethoxy-2-phenylacetophenone (e.g., IRGACURE651, available from Ciba Specialty Chemical);bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide (IRGACURE 819);(2,4,6-trimethylbenzoyl)diphenyl phosphine oxide (LUCERIN TPO, availablefrom BASF (Munich, Germany));ethoxy(2,4,6-trimethylbenzoyl)phenylphosphine oxide (LUCERIN TPO-L fromBASF); and combinations thereof.

The total photoinitiator content of the primary curable composition maybe up to about 10 wt %. Desirably, the total photoinitiator content ofthe primary curable composition is between about 0.5 wt % and about 6 wt%. The photoinitiator component of the primary curable composition mayconsist of a single photoinitiator; alternatively, two or morephotoinitiators may be combined to lend a desired curing property to theprimary curable composition. For example, a combination of IRGACURE 819and IRGACURE 184 may be used to ensure adequate surface cure as well ascomplete cure of the bulk primary coating material.

The photoinitiator, when used in a small but effective amount to promoteradiation cure, provides reasonable cure speed without causing prematuregelation of the coating composition. A preferred dosage for coatingthicknesses of about 25-35 μm is, for example, less than about 1.0J/cm², preferably less than about 0.5 J/cm².

As used herein, the weight percent of a particular component in acurable composition refers to the amount introduced into the bulkcurable composition excluding an additional adhesion promoter and otheradditives. The amount of additional adhesion promoter and various otheradditives that are introduced into the curable composition to produce acomposition of the present invention is listed in parts per hundred. Forexample, a monomer, oligomer, and photoinitiator are combined to formthe curable composition such that the total weight percent of thesecomponents equals 100 percent. To this bulk curable composition, anamount of an additional adhesion promoter, for example 1.0 part perhundred, can be employed in excess of the 100 weight percent of the bulkcomposition.

It may be desirable to include an adhesion promoter in the primarycurable composition. In one embodiment of the present invention, anadhesion promoter is present in the primary curable composition in anamount between about 0.02 to about 10 parts per hundred, more preferablybetween about 0.05 to about 4 parts per hundred, most preferably betweenabout 0.1 to about 2 parts per hundred. In certain embodiments of thepresent invention, the adhesion promoter is present in an amount ofabout 0.1 to about 1 pph. Suitable adhesion promoters includealkoxysilanes, organotitanates, and zirconates. Preferred adhesionpromoters include 3-mercaptopropyltrialkoxysilane (e.g., 3-MPTMS,available from United Chemical Technologies (Bristol, Pa.)),bis(trialkoxysilylethyl)benzene, acryloxypropyltrialkoxysilane (e.g.,(3-acryloxypropyl)-trimethoxysilane, available from Gelest),methacryloxypropyltrialkoxysilane, vinyltrialkoxysilane,bis(trialkoxysilylethyl)hexane, allyltrialkoxysilane,styrylethyltrialkoxysilane, and bis(trimethoxysilylethyl)benzene(available from Gelest (Tullytown, Pa.)); see U.S. Pat. No. 6,316,516,issued Nov. 13, 2001, which is hereby incorporated by reference in itsentirety. The skilled artisan may use other conventional adhesionpromoters in the primary curable compositions used in the presentinvention.

In addition to the above-described components, the primary curablecomposition of the present invention can optionally include any numberof additives, such as reactive diluents, antioxidants, catalysts, andother stabilizers and property-enhancing additives. Some additives canoperate to control the polymerization process, thereby affecting thephysical properties (e.g., modulus, glass transition temperature) of thepolymerization product formed from the primary curable composition.Others can affect the integrity of the polymerization product of theprimary curable composition (e.g., protect against depolymerization oroxidative degradation). For example, the primary curable composition mayinclude a carrier, as described in U.S. Pat. Nos. 6,326,416 and6,539,152, each of which is hereby incorporated herein by reference.

The primary coating composition may also include a strength additive, asdescribed in U.S. patent application Ser. No. 10/077,166, which ishereby incorporated herein by reference in its entirety. Desirablestrength additives include mercapto-functional compounds, such asN-(tert-butoxycarbonyl)-L-cysteine methyl ester; pentaerythritoltetrakis(3-mercaptopropionate); (3-mercaptopropyl)-trimethoxysilane;(3-mercaptopropyl)trimethoxysilane; and dodecyl mercaptan. The strengthadditive is desirably present in the primary curable composition in anamount less than about 1 pph. More desirably, the strength additive ispresent in the primary curable composition in an amount less than about0.5 pph. In certain embodiments of the invention, the strength additiveis present in the primary curable composition in an amount between about0.01 pph and about 0.1 pph.

A preferred antioxidant is thiodiethylenebis(3,5-di-tert-butyl)-4-hydroxyhydrocinnamate) (e.g., IRGANOX 1035,available from Ciba Specialty Chemical).

The composition can further include additional additives such as waxes,lubricants, slip agents, as well as other additives known in the art.

Certain additives may be useful in providing primary curablecompositions having decreased gel times and low moduli. For example, itmay be desirable for the skilled artisan to include in the primarycurable composition an optical brightener, such as UVITEX OB, availablefrom Ciba; Blankophor KLA, available from Bayer; bisbenzoxazolecompounds; phenylcoumarin compounds; and bis(styryl)biphenyl compounds.The optical brightener is desirably present in the primary curablecomposition at a concentration of 0.005-0.3 pph. It may also bedesirable to include in the primary curable composition an aminesynergist, such as triethanolamine; 1,4-diazabicyclo[2.2.2]octane(DABCO); methyldiethanolamine; and triethylamine. The amine synergist isdesirably present in the primary curable composition at a concentrationof 0.02 pph - 0.5 pph.

In coated optical fiber 20 of FIG. 1, primary coating 24 is surroundedby secondary coating 26. While in FIG. 1, the secondary coating is shownas being applied directly to the primary coating, the skilled artisanwill recognize that in alternative embodiments of the invention theremay be one or more intermediate coating layers deposited between theprimary coating and the secondary coating. Secondary coating 26 isformed from a cured polymeric material, and typically has a thickness inthe range of 20-35 μm (e.g., about 27.5 μm). The secondary coatingdesirably has sufficient stiffness to protect the optical fiber; isflexible enough to be handled, bent, or spooled; has low tackiness toenable handling and prevent adjacent convolutions on a spool fromsticking to one another; is resistant to water and chemicals such asoptical fiber cable filling compound; and has adequate adhesion to thecoating to which it is applied (e.g., the primary coating). While thecoated optical fiber 20 of FIG. 1 includes a secondary coating, theskilled artisan will appreciate that the coated optical fibers of thepresent invention need not have a secondary coating; they may include anoptical fiber and a primary coating, but lack a secondary coating.Suitable secondary coatings may be found for example, in U.S. patentapplication Ser. No. 10/840,454, entitled “OPTICAL FIBER COATING SYSTEMAND COATED OPTICAL FIBER”; and U.S. patent application Ser. No.10/454,984, entitled “COATED OPTICAL FIBER, METHOD FOR MAKING COATEDOPTICAL FIBER, AND CURABLE COMPOSITIONS FOR COATING OPTICAL FIBER,” eachof which is hereby incorporated herein by reference in its entirety.Secondary curable compositions having low oligomer content are describedin more detail in U.S. patent application Ser. No. 09/722,895, which isincorporated herein by reference in its entirety. Other suitablematerials for use in secondary coating materials, as well asconsiderations related to selection of these materials, are well knownin the art and are described in U.S. Pat. Nos. 4,962,992 and 5,104,433to Chapin, which are hereby incorporated herein by reference.

Another embodiment of the present invention relates to a method ofmaking an optical fiber including the primary coating describedhereinabove. This method can generally be performed by standard methodswith the use of a coating system of the present invention. For example,a method according to one embodiment of the present invention includesthe steps of providing a bare optical fiber (e.g., fabricated usingmethods familiar to the skilled artisan), coating the optical fiber witha primary curable composition having a gel time less than about 1.4seconds at a UV intensity of 3.4 mW/cm²; and curing the primary curablecomposition to form a primary coating encapsulating the optical fiber,the primary coating having a Young's modulus less than about 5 MPa. Inorder to provide an optical fiber having both primary and secondarycoatings, it may be desirable to apply a secondary curable compositionto the coated glass fiber, and polymerize the secondary curablecomposition to form the secondary coating of the optical fiber.Optionally, the secondary curable composition can be applied to thecoated fiber before polymerizing the primary curable composition, inwhich case only a single polymerization step is employed.

The primary and secondary curable compositions are coated on an opticalfiber using conventional processes, for example, on a draw tower. It iswell known to draw glass optical fibers from a specially prepared,cylindrical preform which has been locally and symmetrically heated to atemperature, e.g., of about 2000° C. As the preform is heated, such asby feeding the preform into and through a furnace, a glass optical fiberis drawn from the molten material. One or more curable compositions areapplied to the glass fiber after it has been drawn from the preform,preferably immediately after cooling. The curable compositions are thencured to produce the coated optical fiber. The method of curing can bethermal, chemical, or radiation induced, such as by exposing the applied(and uncured) curable composition on the glass fiber to ultravioletlight, actinic radiation, microwave radiation, or electron beam,depending upon the nature of the coating composition(s) andpolymerization initiator being employed. It is frequently advantageousto apply both a primary curable composition and any secondary curablecompositions in sequence following the draw process. One method ofapplying dual layers of curable compositions to a moving glass fiber isdisclosed in U.S. Pat. No. 4,474,830 to Taylor, which is herebyincorporated by reference. Another method for applying dual layers ofcurable compositions onto a glass fiber is disclosed in U.S. Pat. No.4,585,165 to Rannell et al., which is hereby incorporated by reference.Of course, the primary curable composition can be applied and cured toform the primary coating material, then the secondary curablecomposition can be applied and cured to form the cured polymericmaterial of the secondary coating.

The coated optical fibers of the present invention are suitable for usein optical fiber ribbons and cables. As such, another embodiment of thepresent invention relates to an optical fiber ribbon including at leastone coated optical fiber as described hereinabove. Another embodiment ofthe present invention relates to an optical fiber cable including atleast one coated optical fiber as described hereinabove.

Another embodiment of the present invention relates to a curablecomposition having a gel time less than about 1.4 seconds at a UVintensity of 3.4 mW/cm², wherein a substantially cured reaction productof the curable composition has a Young's modulus less than about 5 MPa.The curable composition according to this embodiment of the invention issubstantially as described above with respect to the primary curablecomposition used to make the coated optical fibers of the presentinvention. In one especially desirable embodiment of the invention, thecurable composition comprises a polyether or polyester urethane(meth)acrylate oligomer, and a monofunctional epoxy acrylate, desirablyin the concentrations described above. The curable composition may alsoinclude an optical brightener and/or an amine synergist, also desirablyin the concentrations described above.

EXAMPLES

The present invention is further described by the following non-limitingexamples.

Example 1

Primary curable compositions 1-8 and comparative primary curablecomposition C1 were formulated using a high-speed mixer in anappropriate container heated to 70° C. with a heating band or heatingmantle. In each case, the components were weighed into the containerusing a balance and allowed to mix until the solid components werethoroughly dissolved and the mixture appeared homogeneous. Curablecompositions are formulated such that the amounts of oligomer, monomer,and photoinitiator total 100 wt %; other additives are added to thetotal mixture in units of pph. BR3731, BR3741 and BR 582 are oligomersavailable from Bomar Specialties. PHOTOMER 4003 is an ethoxylatednonylphenol acrylate monomer available from Cognis. N-vinylpyrrolidinone and N-vinyl caprolactam are available from Aldrich. CN130is a lauryloxyglycidyl acrylate monomer available from Sartomer Company.IRGACURE 819 and IRGACURE 1850 are photoinitiators available from CibaSpecialty Chemical. IRGANOX 1035 is an antioxidant available from Ciba.Bis(trimethoxysilylethyl)benzene and 3-acryloxypropyltrimethoxysilaneare an adhesion promoters available from Gelest. PHOTOMER 4399 is adipentaerythritol monohydroxy pentaacrylate monomer available fromCognis. UVITEX OB is an optical brightener available from Ciba.Triethanolamine is available from Fisher Scientific. The oligomer andmonomer(s) were blended together for at least one hour at 70° C.Photoinitiator(s) and additives were then added, and blending wascontinued for one hour. Finally, after cooling to room temperature theadhesion promoter was added, and blending was continued for 30 minutes.The components used to formulate primary curable compositions 1-8 andcomparative primary curable composition C1 are detailed below inTable 1. TABLE 1 Primary Curable Composition Component 1 2 3 4 5 6 7 8C1 BR 3731 (wt %) 52 52 52 52 0 0 0 0 0 BR 3741 (wt %) 0 0 0 0 52 52 5252 0 BR 582 (wt %) 0 0 0 0 0 0 0 0 55 PHOTOMER 4003 (wt %) 26 26 26 2625 40 25 40 0 CN130 (wt %) 20 20 20 20 0 0 0 0 42 N-vinyl pyrollidinone(wt %) 0 0 0 0 20 5 0 0 0 N-vinyl caprolactam (wt %) 0 0 0 0 0 0 20 5 0IRGACURE 1850 (wt %) 0 0 0 0 0 0 0 0 3 IRGACURE 819 (wt %) 2 2 2 2 1.51.5 1.5 1.5 0 IRGACURE 184 (wt %) 0 0 0 0 1.5 1.5 1.5 1.5 0 IRGANOX 1035(pph) 0 0 0 0 1 1 1 1 1 bis(trimethoxysilylethyl)benzene (pph) 0 0 0 0 00 0 0 1 3-acryloxypropyltrimethoxysilane (pph) 0 0 0 0 1 1 1 1 0PHOTOMER 4399 (pph) 0 0 5 5 0 0 0 0 0 UVITEX OB (pph) 0 0.05 0 0 0 0 0 00 triethanolamine (pph) 0 0 0 0.1 0 0 0 0 0

Primary curable compositions 1-8 and comparative primary curablecomposition C1 were cured into films for testing of mechanicalproperties. Wet films were cast on silicone release paper with the aidof a draw-down box having an about 0.005″ gap thickness. Films werecured using a Fusion Systems UV curing apparatus with a 600 W/in D-bulb(50% power, 10 ft/min belt speed, nitrogen purge) to yield primarycoatings 1-8 and comparative primary coatings C1 in film form. Curedfilm thickness was between about 0.003″ and 0.004″.

The films were allowed to age (23° C., 50% relative humidity) for atleast 16 hours prior to testing. Film samples were cut to a specifiedlength and width (about 15 cm × about 1.3 cm). Young's modulus, tensilestrength at break, and elongation at break were measured using a Sintechtensile tester. Films were tested at an elongation rate of 2.5 cm/minstarting from an initial jaw separation of 5.1 cm. Glass transitiontemperatures of the cured films were determined by determining the peakof the tan 6 curves measured on a Seiko-5600 DMS in tension at afrequency of 1 Hz. Thermal and mechanical properties (tested inaccordance with ASTM 82-997) of the cured films are reported in Table 2,below. TABLE 2 Primary Young's modulus Tensile Strength elongation atCoating (MPa) (MPa) break (%) T_(g) (° C.) 1 1.68 0.66 76 −27.1 2 1.680.69 79 −28.7 3 4.80 4.39 78 −11.7 4 ND ND ND −13.8 5 0.77 0.58 ND ND 60.65 0.62 ND −26 7 0.64 0.49 ND −15 8 0.67 0.53 ND −26 C1 5.42 1.50 43−7.0ND = not determined

Gel times and spectroscopic cure speeds for primary curable compositions1-8 and comparative primary curable composition C1 were determined usingthe test methods described above. Gel times at UV intensities of 3.4mW/cm² and 8.5 W/cm² are given in Table 3, below. Gel times are averagesof two runs, and spectroscopic cure speeds are averages of two runs.TABLE 3 Spectroscopic 3.4 mW/cm² 8.5 mW/cm² cure speed Primary Gel Std.Gel Std. Cure Std. Curable time dev time dev. speed dev. Composition(sec) (sec) (sec) (sec) (%/sec) (%/sec) 1 1.27 0.05 0.52 0.05 177 2 21.02 0.05 0.43 0.05 188 2 3 1.17 0.08 0.55 0.13 223 10 4 1.04 0.05 0.450.07 223 15 5 ND ND 0.20 0.05 ND ND 6 ND ND 0.31 0.05 ND ND 7 ND ND 0.240.05 ND ND 8 ND ND 0.42 0.05 ND ND C1 2.28 0.08 0.96 0.04 207 4

While the cure speed of comparative primary coating composition C1 iscomparable to that of primary curable compositions 1-8, the gel times ofprimary curable compositions 1-8 are shorter than that of comparativeprimary coating composition C1. The optical brightener andtriethanolamine additives appear to be effective at shortening the geltime.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present inventionwithout departing from the spirit and scope of the invention. Thus, itis intended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1. A coated optical fiber comprising: an optical fiber having a core anda cladding; and a primary coating encapsulating the optical fiber, theprimary coating having a Young's modulus of about 5 MPa or less, theprimary coating being the cured reaction product of a primary curablecomposition having a gel time less than about 1.4 seconds at a UVintensity of 3.4 mW/cm².
 2. The coated optical fiber of claim 1, whereinthe primary curable composition has a gel time less than about 0.6seconds at a UV intensity of 8.5 mW/cm².
 3. The coated optical fiber ofclaim 1, wherein the primary curable composition has a gel time lessthan about 0.45 seconds at a UV intensity of 8.5 mW/cm².
 4. The coatedoptical fiber of claim 1, wherein the primary curable composition has agel time less than about 1.2 seconds at a UV intensity of 3.4 mW/cm². 5.The coated optical fiber of claim 1, wherein the primary coating has aYoung's modulus less than about 2 MPa.
 6. The coated optical fiber ofclaim 1, wherein the primary curable composition has a spectroscopiccure speed of at least about 150%/second as determined by FTIR.
 7. Thecoated optical fiber of claim 1, wherein the primary curable compositioncomprises a polyether or polyester urethane (meth)acrylate oligomer. 8.The coated optical fiber of claim 1, wherein the primary curablecomposition comprises a monofunctional epoxy acrylate.
 9. The coatedoptical fiber of claim 1, wherein the primary curable compositioncomprises a multifunctional (meth)acrylate.
 10. The coated optical fiberof claim 1, wherein the primary curable composition includes an N-vinylamide.
 11. The coated optical fiber of claim 1, wherein the primarycurable composition comprises an optical brightener.
 12. The coatedoptical fiber of claim 1, wherein the primary curable compositioncomprises an amine synergist.
 13. The coated optical fiber of claim 1,wherein the primary coating has a glass transition temperature morenegative than about −10° C.
 14. The coated optical fiber of claim 1,further comprising a secondary coating encapsulating the primarycoating.
 15. An optical fiber ribbon comprising at least one of thecoated optical fibers of claim
 1. 16. An optical fiber cable comprisingat least one of the coated optical fibers of claim
 1. 17. A method forcoating an optical fiber comprising the steps of: providing a bareoptical fiber; coating the optical fiber with a primary curablecomposition having a gel time less than about 1.4 seconds at a UVintensity of 3.4 mW/cm²; and curing the primary curable composition toform a primary coating encapsulating the optical fiber, the primarycoating having a Young's modulus less than about 5 MPa.
 18. The methodof claim 17, wherein the coating and curing steps are performed at aprocess speed of greater than about 20 m/s.
 19. The method of claim 17,wherein the primary curable composition has a spectroscopic cure speedof at least about 150%/second as determined by FTIR.
 20. A curablecomposition having a gel time less than about 1.4 seconds at a UVintensity of 3.4 mW/cm², wherein a substantially cured reaction productof the curable composition has a Young's modulus less than about 5 MPa.21. The curable composition of claim 20, wherein the curable compositioncomprises a polyether or polyester urethane (meth)acrylate oligomer, anda monofunctional epoxy acrylate.
 22. The curable composition of claim20, wherein the curable composition includes an optical brightener. 23.The curable composition of claim 20, wherein the curable compositionincludes an amine synergist.
 24. The curable composition of claim 20,wherein the curable composition includes an N-vinyl amide.